1. Field of the Invention
The present invention relates to a digital polarimetric system and, more particularly, to a digital polarimeter for controlling the generation of polarization agile signals in a signal transmission system.
2. Description of the Related Art
Ground-based and airborne radar systems have long been used to detect and track hostile targets, such as aircraft and missiles. Typically, a radar system transmits RF signals, such as a sequence of pulses, toward the target. The surface of the target reflects at least some portion of the incident signal energy back toward the radar antenna where the pulse echo is detected, allowing the radar system to determine the target""s range, angle or velocity.
To counter the effectiveness of such radar systems, electronic warfare (EW) systems attempt to interfere with radar signals by generating electronic counter measure (ECM) signals designed to confuse, mislead or overwhelm the radar system. By accurately determining the polarization of an arriving radar signal, an EW system can transmit a signal with the same polarization back to the radar system to interfere with the echo signal, or transmit a signal with an orthogonal polarization to deceive or jam the radar system. EW systems have relied on polarimeters to accurately measure the polarization of received radar signals and to control the polarization of the ECM signals transmitted back to the radar system.
Modern EW systems typically include a dual polarized antenna capable of receiving and transmitting RF signals with identifiable horizontal and vertical signal components. Such systems employ two polarimeters: a receive polarimeter, involving a four terminal network inserted into the receiver line; and a transmit polarimeter embedded in the input lines to a pair of power amplifiers which supply the components of signals to be transmitted to input terminals of the dual polarized antenna. The receive polarimeter measures the polarization of intercepted signals and generates two phase signals which represent the determined polarization. The transmit polarimeter receives the phase signals from the receive polarimeter and controls the polarization of a transmitted signal in accordance therewith, thereby producing a polarization agile signal that correspond to the determined polarization of the intercepted signal. Essentially, the transmit polarimeter is an analog network device that accepts one input RF signal and produces two signal components with a specific amplitude and phase relationship determined by the parameters supplied by the receive polarimeter.
EW system technology development was in its infancy at the time polarimeters in currently operational systems were developed; consequently, present polarimeter designs are based on analog components. These analog-based designs result in polarimeters that are bulky, heavy, costly, and that dissipate significant RF signal power. Although polarimeters based on digital technology could potentially avoid these problems, the extraordinarily high digital sampling rates that would be required to accurately measure and apply polarization are not feasible with current technology.
A receive digital polarimeter that overcomes this barrier to use of digital polarimeters is described in U.S. patent application Ser. No. 09/711,168, entitled xe2x80x9cDigital Polarimetric System,xe2x80x9d filed Nov. 13, 2000, hereby incorporated herein by reference in its entirety. The system described therein includes a digital polarimeter that employs a signal time stretching technique and digital signal processing of the time-stretched signal to accurately measure the polarization of a received RF signal with commercially available digital hardware. A superheterodyne receiver down converts received RF signal components to an intermediate frequency (IF), and analog-to-digital converters sample the signal components at much lower sampling rates than would otherwise be required to accurately measure the signal polarization. Each signal sample is xe2x80x9ctime stretchedxe2x80x9d by storing each sample in M locations in a memory, such that N samples occupy Mxc3x97N memory locations. A digital signal processor applies incremental phase shifts to the digital samples until a phase-shifted combination of the digital samples yields a minimum null output. The phase shifts producing the minimum null identify the polarization of the received signal. The stretching and digital processing yield the required number of samples per cycle of the received signal for accurate polarization measurement, thus effectively increasing the digital sampling frequency.
The aforementioned system produces highly accurate polarization measurements with currently available analog-to-digital converters. By using commercially available digital hardware, the digital implementation makes polarimetric systems affordable to more EW systems than the more expensive conventional analog polarimeters. Furthermore, the use of digital polarimeter techniques avoids potential performance degradation caused by cross leakage signals typically present in conventional analog polarimeters, is more compact, lower in weight, more power efficient, and offers a greater mean time between failures than conventional high-power analog polarimeters.
While this digital receive polarimeter provides many advantages, it would be beneficial if similar advantages could be realized in a transmit polarimeter. Without improvements with respect to the transmit polarimeter, designs that include conventional analog transmit polarimeters may remain unaffordable in many small or low-cost EW systems. Furthermore, operational EW systems will remain burdened with the disadvantages of the more expensive, less reliable, bulky, heavy, high power consumption, analog transmit polarimeters. Accordingly, there remains a need for a reliable, inexpensive and compact transmit polarimetric system that uses currently available digital technology to overcome the performance limitations of analog transmit polarimeters.
Therefore, in light of the above, and for other reasons that become apparent when the invention is fully described, an object of the present invention is to employ digital polarimetric techniques to control the generation of polarization agile transmission signals using commercially available digital hardware.
A further object of the present invention is to reduce manufacturing and maintenance costs of polarimetric systems.
Yet a further object of the present invention is to eliminate performance degradation caused by electromagnetic signal leakage in transmission systems that employ polarimetric devices.
A still further object of the present invention is to reduce power requirements associated with transmission systems that employ polarimetric devices.
Another object of the present invention is to reduce the size and weight of polarimetric systems.
Yet another object of the present invention is to increase the reliability of transmission systems that employ transmit polarimetric devices, for example, increased mean time between failures (MTBF).
It is a further object of the present invention to make practical the delivery of polarization signal data to a remotely located transmitter.
The aforesaid objects are achieved individually and in combination, and it is not intended that the present invention be construed as requiring two or more of the objects to be combined unless expressly required by the claims attached hereto.
In accordance with the present invention, a digital polarimeter employs a signal time stretching technique to apply polarization phase shift characteristics to digitally generated polarization agile signal components using commercially available digital hardware. An RF signal to be transmitted is down-converted to an intermediate frequency (IF) and digitally sampled at a rate feasible with currently-available analog-to-digital converters. Horizontal and vertical signal components of the sampled IF signal are generated, and the digital samples of the two signal components are stored in memory. Each digital signal sample is xe2x80x9ctime stretchedxe2x80x9d by storing each sample in M memory locations, such that N samples occupy Mxc3x97N memory locations. Because each sample is stored in M memory locations, each incremental phase shift from one memory location to the next corresponds to 1/Mth of the actual sampling interval. The stretching process effectively increases the digital sampling frequency and provides the number of samples per cycle and, hence, phase resolution required to accurately apply high resolution polarization characteristics to the transmit signal.
At stages of the digital polarimeter processing associated with the generation of polarization agile signal components, phase shifts analogous to those applied in a conventional analog transmit polarimeter are applied to the digital sample streams. In effect, the phase shifts required to produce the desired polarization are carried out by shifting mathematical operations performed on stored digital samples (i.e., additions and subtractions of samples in particular memory locations) by a certain number of memory locations that corresponds to a desired phase shift. The number of memory location shifts required to implement a given phase shift is directly related to the number of digital samples/cycle that are used to represent one cycle of the analog signal. Accordingly, where the digital polarimeter system receives phase shift information in analog form, a translation process is used to calculate the corresponding number of memory location phase shifts to be applied to the digital signal samples. In the case where phase shift information is generated by a digital receive polarimeter as a determined number of memory location shifts, the shift values can be directly applied by the transmit polarimeter of the present invention to effect a particular transmit polarization. Once the transmit polarimeter has applied the desired phase shifts, the transmit signal components are time compressed, converted from digital to analog signals, up converted to the original RF carrier frequency and supplied to an antenna for transmission.
Implementation of the time stretching technique makes the digital transmit polarimeter feasible by providing the necessary resolution required to apply highly accurate polarization phase shift characteristics to digitally generated polarization agile signal components. Using commercially available digital hardware, the digital implementation of the transmit polarimeter makes polarimetric systems affordable to more EW systems than the more expensive conventional analog transmit polarimeters. The digital implementation also avoids potential performance degradation caused by cross leakage signals typically present in conventional analog polarimeters. Relative to conventional high-power analog transmit polarimeters, the digital polarimeter of the present invention is more compact, lower in weight and more power efficient. The digital transmit polarimeter of the present invention also offers the potential of a more reliable system, with a greater mean time between failures. More generally, the time stretching technique of the present invention can be applied to a digitized input signal to manipulate the signal phase and amplitude in virtually any manner before being xe2x80x9creconstitutedxe2x80x9d in the form of a polarization agile transmission signal. Furthermore, polarization control information can be received by the transmit polarimeter of the present invention from a remote receive polarimeter or controller via data links. This capability provides great flexibility not only with respect to the characteristics of the signal to be transmitted, but also with respect to the location, and/or multiple locations, from which the polarization agile signal can be transmitted.
The above and still further objects, features and advantages of the present invention will become apparent upon consideration of the following definitions, descriptions and descriptive figures of specific embodiments thereof. While these descriptions go into specific details of the invention, it should be understood that variations may and do exist and would be apparent to those skilled in the art based on the descriptions herein.